The present invention relates to an orthopedic implant used in arthroplasty of the fingers, comprising a first element for a proximal phalanx and a second element for an adjacent distal phalanx.
A proximal phalanx is to be understood to mean a phalanx located on the body side or member side (hand or foot) in question, and a distal phalanx is to be understood to mean a phalanx located toward the outside of this member, according to the orientation rules conventionally adopted in anatomy that will moreover be used below.
The invention also relates to a method for installing such an implant.
The invention finds one particularly important, although not exclusive, application in the medical field of proximal interphalangeal joint prostheses for the hand (known in the field by the initials PIP).
However, it can also and in particular be used advantageously for interphalangeal finger joint prostheses for the thumb (thumb IP) or for distal interphalangeal prostheses (DIP) for the hand or foot, by simple geometric adaptation to the bone sites in question.
There are already known interphalangeal joint prostheses comprising first and second elements, each element having a stem for implantation in the bone and a complementary interphalangeal pivot head.
For example, there is known (EP 1 339 362) [U.S. Pat. No. 6,699,292] an implant comprising a first element provided with a head having a convex bicondylar pivot surface having a central valley, and a second element provided with a complementary head having a biconcave pivot surface designed to cooperate with the convex surface so that there is a congruence between these surfaces in the sagittal plane.
Congruence is to be understood to mean the fact of coinciding or of fitting perfectly with one another in the sagittal plane.
The congruence in the sagittal plane therefore means that the point of contact between the two articular surfaces in this plane always passes through the same line of surface contact during movement.
It should be recalled that a sagittal or parasagittal plane is a plane parallel to the median plane when the human body is in the upright position.
Such an arrangement, although it allows good guidance of the joint, is sometimes prejudicial to the service life of the prosthesis. This is because it may give cause jamming and thus a failure of the prosthesis in up to 20% of cases, depending on the surgeon carrying out the procedure; in addition, it causes significant wear on the articular surfaces of the prosthesis, requiring a new operation.
Prostheses are also known (EP 0 572 339) [U.S. Pat. No. 5,405,399] having barrel-shaped bicondylar surfaces with different radii of curvature.
Here again, this type of implant that is used in particular as a shoulder prosthesis, poses problems of wear and is moreover not very suitable for joints of small size such as those of the fingers.
In order to attempt to alleviate these drawbacks, prostheses made from bioceramic, for the fingers or toes, have been envisaged (DE 198 207 48).
Although such prostheses are more resistant to wear, they nevertheless present a risk of shattering and are also complex to install.